Hardness testers are instruments which are well known to the skilled technician. The apparatus, which may take different forms, comprises an indenter constituted essentially by a stylus assembly which may assume various forms depending on the type of sample to be tested provided with an indenting tip of a hard and undeformable material, commonly a diamond or a hard metal, and a loading system for applying a certain load. The hardness assessment is generally a multistep test whereby the size of the indentation produced by the indenting tip on the surface of the piece to be tested under a certain load applied to the indenter, or the depth of penetration of the indenter is measured.
The tip of the indenter may have a conical, a spherical (Brinnel) or square piramidal (Vikers) form. The diamond or the hard metal are the materials commonly used for making these tips which must be sufficiently hard so as not to be scratched by the material being tested and undeformable under the test load.
A bench-type apparatus comprises usually a support structure, a loading system through which it is possible to load and by an inventory of implements for permitting to select a certain configuration of the indenter, suitable for the piece to be tested. e.g. for effecting tests on external or internal surfaces of a machined piece of machinery.
Where the indentation cannot be viewed through an optical microscope, for example in the case of an internal surface of a tubular piece, the assessment of the hardness must necessarily be based upon a micrometric determination of the depth of penetration of the indenter tip in the material being tested, according to the capabilities of the known apparatuses. These measurements of size and/or depth of an indentation are burdensome and are often a source of evaluation errors.
In U.S. Pat. No. 4,848,141 and in the Article "Measurements of Hardness at Indentation Depths as Low as 20 Nanometers"; by W. C. Oliver, R. Hutchings and J. B. Pethica, pp 90-108, ASTM Special Technical Publication No. 889, 1986, a method of determining hardness, yield strength, and other mechanical parameters of a material are disclosed wherein the load applied to a microindenter and the displacement of the indenter after the establishment of contact with the surface of the sample are electronically measured and elaborated to obtain the value of the particular mechanical parameter.
On the other hand, the contact area between two bodies is difficult to determine when the area of contact is less than a few square microns. There have been prior attempts to obtain such a measurement by determining the electrical resistance at the junction. However, to date, such a technique has never found application in indenter-type hardness testing apparatus which for other reasons must utilize an intrinsically insulating indenter tip, such as a diamond tip.